A workpiece such as a printed substrate or an electronic component used in electronic equipment is typically soldered by the soldering iron method, the flow soldering method, or the reflow soldering method. In the soldering iron method, soldering is performed by placing a flux-cored wire solder on a portion to be soldered of a workpiece and then heating the wire solder from above by a soldering iron until the flux-cored wire solder is melted. In the soldering iron method, soldering is carried out at one location to be soldered at a time, so it is not suited to mass production, but it is suited for soldering electronic parts having low heat resistance to a workpiece on which soldering has been performed by other soldering methods or for repairing soldering defects which occur with other soldering methods.
In the flow soldering method, a printed wiring board having electronic parts placed thereon is contacted with molten solder. This method has excellent productivity in that many locations of the board can be soldered at the same time. However, in the flow soldering method, since the entirety of a printed wiring board is brought into contact with molten solder, it has problems in that solder adheres to unnecessary locations and molten solder at a high temperature directly contacts electronic parts, thereby causing thermal damage to electronic parts.
In the reflow soldering method, soldering is carried out by placing solder only on necessary locations of a workpiece and then heating the solder with a heating apparatus such as a reflow furnace, an infrared radiation apparatus, or a laser apparatus. Therefore, it not only has excellent productivity but also excellent reliability of soldering in that solder does not adhere to unnecessary locations. Accordingly, the reflow soldering method is much employed for soldering of today's workpieces which require high reliability.
Solders which are used for the reflow soldering method include solder pastes and solder preforms. A solder paste is obtained by mixing a viscous flux and solder powder, and it is applied to the portions to be soldered of a workpiece by printing or with a dispenser. A flux used in a solder paste contains flux components such as a rosin, an activator, and a thixotropic agent dissolved in a solvent. As a result, soldering with a solder paste always leaves flux residue. During soldering with a solder paste, the activator, the thixotropic agent, the solvent, or similar components in the paste almost entirely vaporize, but they do not completely vaporize, and a slight amount thereof remains in the flux residue. The flux components tend to absorb moisture, and as the flux components remaining in the flux residue absorb moisture from the atmosphere, they sometimes form corrosive products on or around the soldered joint or decrease the insulation resistance of the soldered joint. Therefore, a solder paste is not suitable for soldering of a workpiece requiring high reliability.
Thus, a solder preform which can be used for soldering without using a flux is suitable for soldering of workpieces requiring high reliability. A solder preform is a pellet or a washer having a shape suited for the portion to be soldered. In reflow soldering using a solder preform, the solder preform is placed on a workpiece and then heated in a reducing atmosphere such as a hydrogen gas atmosphere. When a workpiece having a solder preform placed thereon is heated in a hydrogen gas atmosphere, the hydrogen reduces and removes oxides formed on the surface of the workpiece and the solder preform and enables them to be wet by molten solder.
For a workpiece requiring high reliability, such as one soldered by die bonding, since a solder paste which leaves a flux residue cannot be employed, a solder preform is used. In die bonding, a substrate and a semiconductor chip are joined with solder. Soldering is carried out by placing a solder preform between the substrate and the semiconductor chip followed by heating in a reducing atmosphere.
When a solder preform and a semiconductor chip are placed on a substrate and heated, as the solder preform melts, the molten solder may sometimes be protruded from the area between the portions being soldered due to pressing by the weight of the semiconductor chip, thereby decreasing the amount of solder present between the portions being soldered. Bonding by means of soldering can afford a sufficient bonding strength provided that an adequate amount of solder is present between portions being soldered. However, if solder is protruded from the area between portions being soldered by the weight of a semiconductor chip which is placed on top of it as is the case in die bonding, the bonding strength ends up becoming weak.
In order to provide a suitable clearance between portions being soldered and to maintain a suitable amount of solder between the portions being soldered, in the prior art, a plurality of particles or spheres of a high-melting metal (referred to below simply as metal particles) having a higher melting point than solder, such as Ni, Cu, Ag, Fe, Mo, or W, have been interposed between portions being soldered. Separately placing the metal particles between the portions being soldered at the time of soldering is extremely time consuming and results in poor efficiency. Therefore, solder preforms already having metal particles dispersed therein have been used.
Methods of producing solder preforms having metal particles dispersed therein include the pressing method and the melting method. In the pressing method, a large number of metal particles are placed atop a single solder sheet, and the metal particles are embedded in the solder sheet by passing the sheet between a pair of rollers (Patent Document 1), or metal particles are sandwiched between two solder sheets and then punched out with a press (Patent Document 2).
In the melting method, metal particles are dispersed in molten solder, the molten solder is then cast into a mold to form a billet, the billet is formed into a solder sheet by extrusion, and then the solder sheet is punched out with a press (Patent Document 3). In the melting method described in Patent Document 3, the surfaces of metal particles are subjected to electroplating or electroless plating. Next, a mixture of the metal particles and a flux is put into molten solder and stirred, and then the molten solder is cast into a mold to form a billet. The billet is then rolled to form a sheet, which is then shaped by a press into solder preforms having a predetermined shape.
Patent Document 1: JP 03-281088 A1
Patent Document 2: JP 06-285686 A1
Patent Document 3: JP 06-31486 A1